1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to frequency modulated (FM) radio receivers used in such systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel, or channels, (e.g., one or more of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel, or channels. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter converts data into RF signals by modulating the data in accordance with the particular wireless communication standard to produce baseband signals and mixes the baseband signal with a local oscillation in one or more intermediate frequency stages to produce the RF signals. The radio receiver generally includes an antenna section, a filtering section, a low noise amplifier, an intermediate frequency (IF) stage, and a demodulator. The antenna section receives RF signals and provides them to the filtering section, which, in turn, passes RF signals of interest to the low noise amplifier. The low noise amplifier amplifies the received RF signals of interest and provides them as amplified RF signals to the IF stage. The IF stage steps down the frequency of the RF signals of interest to an intermediate frequency or to base-band. The IF stage provides the intermediate frequency signals or base-band signals to the demodulator, which recaptures the data in accordance with the demodulation protocol.
For the demodulator to accurately recover data from IF signals or baseband signals, DC offsets must be overcome. As is known, in frequency modulated (FM) systems, one source of DC offsets in the demodulated output result when the clock circuitry of a transmitting radio produces a slightly different clock rate than the clock rate produced by the receiving radio. In other words, the local oscillation within the transmitting radio does not produce the exact same rate of oscillation as the rate produced by the local oscillation in the receiving radio.
To correct for DC offsets, radio receivers include, within the demodulator, a DC offset detection circuit and DC offset compensation circuit. The DC offset detection circuit indicates the level of DC offset while the DC compensation circuit essentially removes the DC offset from the demodulated IF signals or baseband signals. While this technique compensates for DC offset, the frequency offset is present throughout the radio receiver. As is further known, the frequency offset is an error and its presence throughout most of the radio receiver processing can adversely affect the receiving processing function.
Therefore, a need exists for a radio receiver that directly removes the source of the DC offset before it is created.